Oz says: A surprising number of drugs might do it, especially when you first start taking them. "If a prescription is going to make you gain weight, it usually does so in the first six months of treatment," says Holly Lofton, MD, director of the Medical Weight Management Program at NYU Langone Medical Center. Some drugs are well-known for having this side effect — see the list below.

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If you're keeping your other habits the same and feel pretty sure it's your meds' fault, speak up. "In many cases, there are other drugs you could try that don't have an impact on weight," says Lofton. Call your doc's office; the first step is telling her what's happening and that it bothers you. What you shouldn't do: stop taking a med without discussing it with her.

The Classic Culprits

Antidepressants: Some stimulate appetite; others rekindle the desire for food in people who had lost interest (a healthy thing, but it can surprise you on the scale).